1. Field of the Invention
The present invention relates to a magnetic resonance imaging apparatus for transmitting a magnetic resonance signal and a clock signal by radio.
2. Description of the Related Art
In magnetic resonance imaging (MRI) apparatuses, a detection coil for detecting a magnetic resonance signal is placed in the imaging space of a gantry, along with a subject. The magnetic resonance signal detected by the detection coil is transmitted to a main unit (hereinafter referred to as “a control/imaging unit”) via a cable led from the interior of the imaging space to the outside of the gantry. The control/imaging unit executes imaging by performing, on the detected magnetic resonance signal, data processing that includes image reconstruction processing.
In the above-mentioned general structure, the cable is often obstructive to operators and/or operations. To avoid this, a probe unit including a detection coil called an RF probe has been proposed in which a magnetic resonance signal is digitized by an analog-to-digital converter (ADC), then converted into a radio signal, and transmitted to a control/imaging unit.
The ADC in the probe unit requires a sampling clock signal. When the sampling clock signal must be highly accurately synchronized with a reference clock signal used by the control/imaging unit, a highly accurate oscillator, such as an oven controlled crystal oscillator (OCXO) or a temperature controlled crystal oscillator (TCXO), must be used as a clock signal source. Since such a highly accurate oscillator has a large size, if it is provided in the probe unit, the probe unit is inevitably increased in size and weight, which is a significant load on a subject.
JP-A 5-261083 (KOKAI) describes that a sampling clock signal may be transmitted by radio from a control unit to a probe unit. In this case, the problem that occurs when a clock signal source is provided in the probe unit is avoided. However, this publication does not disclose a method of transmitting the sampling clock signal by radio or a method of regenerating the clock signal from a received signal.
On the other hand, JP-A 6-232930 (KOKAI) discloses a clock signal regeneration circuit for use in a demodulator for demodulating an angular modulation wave. This clock signal regeneration circuit regenerates a clock signal when an angular modulation signal, such as a QPSK input to the demodulator, is demodulated. More specifically, the angular modulation wave input to a frequency converter formed of an oscillator, a mixer and a low-pass filter is subjected to frequency conversion, then digitized by an ADC, and then input to a digital signal processor, where demodulation and clock signal regeneration are performed.
JP-A 5-261083 (KOKAI) does not disclose a method of transmitting the sampling clock signal by radio, or a method of regenerating the clock signal from a received signal. If the clock signal regeneration circuit disclosed by JP-A 6-232930 (KOKAI) is used, the frequency converter, the ADC and the digital signal processor must be employed. Further, the accuracy of the regeneration clock signal is influenced by that of the oscillator, and that of the clock signal used in the ADC.
In addition, in general, a regeneration stop zone, in which the regeneration of the clock signal is temporarily stopped, exists in a magnetic resonance signal acquisition period, and there is a case where it is necessary to keep in phase the clock signals generated before and after the regeneration stop zone, or to keep in phase the clock signals even when the distance between the transmitting and receiving antennas varies during the magnetic resonance signal acquisition period. However, neither JP-A 5-261083 (KOKAI) nor JP-A 6-232930 (KOKAI) discloses any countermeasures against these problems.